The invention relates to a plasma panel scanning process adapted to the contents of the video image to be displayed and its associated device.
An elementary cell of a plasma panel can have only two states: unlit and lit. It is known that, since analogue modulation of the amount of light emitted by a pixel is not possible, half-tones are generated by temporal modulation of the duration of emission of the pixel in the image period T. This image period consists of as many sub-periods (To, 2To, . . . , 2nxe2x88x921To) which are multiples of a value To, as there are bits for coding the video (n bits). On the basis of the n sub-periods it is possible, by combination, to reconstruct 2n different grey levels of linearly distributed luminance:       0    ·          L              m        ⁢                  xe2x80x83                ⁢        a        ⁢                  xe2x80x83                ⁢        x              ;            1                        2          n                -        1              ·          L              m        ⁢                  xe2x80x83                ⁢        a        ⁢                  xe2x80x83                ⁢        x              ;            2                        2          n                -        1              ·          L              m        ⁢                  xe2x80x83                ⁢        a        ⁢                  xe2x80x83                ⁢        x              ;  …  ⁢      xe2x80x83    ;                    2                  n          -          2                                      2          n                -        1              ·          L              m        ⁢                  xe2x80x83                ⁢        a        ⁢                  xe2x80x83                ⁢        x              ;      1    ·          L              m        ⁢                  xe2x80x83                ⁢        a        ⁢                  xe2x80x83                ⁢        x            
Lmax is the luminance of the cell when the latter is excited continuously, that is to say during all the sub-periods.
The eye of the observer will integrate, over the duration of the image period T, the various combinations of luminous emissions and in this way recreate the various shades in the grey levels. The lowest luminance level which this method enables us to restore is the value Lmin=Lmax/2nxe2x88x921. This elementary value of luminance depends on the maximum value of the luminance (Lmax) given by the technology of the plasma panel but also on the definition of the video (n). The restoring of the video images may in some cases require high luminance, and in other cases high resolution in the low luminance levels, as is the case in television.
The perception of the grey levels by the observer is characterized by the ratio xcex94L/L referred to as the Weber-Fechner ratio which defines the relative variations in luminance which the eye of the observer can perceive as a function of the luminance values. The way in which this ratio alters as a function of luminance is given in FIG. 1. The abscissa axis represents the logarithmic value of the luminance in cd/M2 and the ordinate axis the logarithmic value of the relative variation in this luminance. This curve is dependent on a parameter, namely the background luminance or ambient luminance, the luminous environment influencing the sensitivity of the eye. For example, the subjective black limit, namely the value of luminance below which the eye no longer distinguishes the shades, depends on this surrounding luminance. For the luminance values of a plasma panel, values lying between about 0.1 cd/m2 and 200 cd/m2 and corresponding to the right-hand part of the curve, this curve will be approximated by a straight line with equation:
log(xcex94L/L)=xe2x88x92a.log(L)+b.
b is an increasing function of the background luminance. In television, the assumption is made that the small image is viewed in a fixed-luminance environment.
If, in the equation, the elementary variation in luminance xcex94L, that is to say the variation in the grey levels which is perceptible to the eye, is replaced by the elementary variation in the grey levels displayed by the plasma panel, that is to say the minimum coding value permitted by n bits and defined in our system by Lmin=Lmax/2nxe2x88x921 (rounded to Lmax/2n), the straight line 1 of FIG. 2 is obtained, with equation:   n  =                              a          -          1                          log          ⁢                      xe2x80x83                    ⁢          2                    ⁢      log      ⁢              xe2x80x83            ⁢      L        +                                        log            ⁢                          xe2x80x83                        ⁢                          L                              m                ⁢                                  xe2x80x83                                ⁢                a                ⁢                                  xe2x80x83                                ⁢                x                                              -          b                          log          ⁢                      xe2x80x83                    ⁢          2                    .      
The abscissa carries the logarithm of the luminance L and the ordinate the value n, that is to say the number of bits for coding the video. This curve 1 thus represents, for a given luminance value L, the number of video bits necessary for obtaining a resolution compatible with the minimum perceptible luminance value. This curve depends on the luminous environment (parameter b).
Thus, the number of bits necessary for coding the luminance, so that the latter is compatible with the luminance variations perceptible to the eye, increases as the luminance to be displayed decreases. Alternatively, in order to be adapted to the possible differentiation by the eye of two neighbouring grey levels, the lower the luminance level displayed, the higher must be the number of bits for coding the video.
This curve 1 corresponds to a luminous environment of greater than 200 lux, that is to say the observation of an image in a strongly lit room. The definition of the video can then be limited, without the quality of the image being overly degraded thereby, which degradation is all the weaker (subjective perception) when the images display very different areas of luminance.
In the case of a relatively weak luminous environment, for example less than 100 lux, curve 1 moves towards curve 2 (b decreasing). The number of bits for coding the video which makes it possible to differentiate all the grey levels then varies between 16 bits for luminance values of 10xe2x88x921 cd/m2 and 12 bits for luminance values of 1 cd/m2. The 8 bits or 10 bits for coding the video become insufficient for good restoration of low luminances. The displaying of a video image coded on 8 or 10 bits gives rise to a lack of details in the image or to black areas in those places where a cathode-ray tube would display weak but non-zero luminances. This phenomenon is particularly striking in respect of scenes exhibiting uniformly dark images.
The purpose of the present invention is to alleviate the abovementioned drawbacks.
To this end, the subject of the invention is a process for scanning cells of a matrix-controlled display for the displaying of grey levels of a video signal, the scan being split up into sub-scans relating to each bit of column control words, characterized in that the video signal is coded on a number of bits greater, by the value p, than the number of sub-scans of the display so as to deliver video coding words, in that an estimate of the contents of the image is made by determining, on a complete image, the number of times for which each of the first p most significant bits (MSB) of the video coding words takes the value one, in that, if these numbers are greater than or equal to specified thresholds, the p least significant bits of the video coding word are ignored in order to perform the coding of the column control words on the basis of the video coding words and, in the case in which this number is less, the p most significant bits are ignored for this coding and the sub-scan relating to these bits and for this image is assigned to the displaying of the item relating to the p least significant bits.
The subject is also a device for scanning a matrix-controlled display comprising a video processing circuit receiving a video signal and delivering video coding words, a scan management circuit linked to the processing circuit, to line supply circuits for selecting lines and to column supply circuits for controlling the columns of the display on the basis of column control words, the scan of a video image consisting of a succession of p sub-scans dependent on the weights of the bits of the column control words, characterized in that the number of bits of the video coding words is greater by a value p than the number of sub-scans s controlled by the scan management circuit, in that the video processing circuit estimates the contents of each image so as to determine the number of one values of each bit from among the p most significant bits of the video coding words for a complete image, in that the scan management circuit controls the transmission to the column control circuits, in a specified order, of the s most significant bits or of the s least significant bits of the video coding words depending on the number of one values and in that, in the latter case the scan management circuit controls the line supply circuit so that it replaces the p sub-scans assigned to the most significant bits (b7) by p sub-scans corresponding to the p least significant bits (bxe2x88x921).
According to the invention, in the case of an image defined by coding words which do not activate the most significant bit, the processing sub-scan relating to this most significant bit, which has no influence on the rendition of the luminance of the image, is allocated to the displaying of an additional item which corresponds to a bit of lower weight than the smallest weight as defined in a conventional scan of a plasma panel, according to the prior art. To do this, the video signal received at the input of the device exploits this item or else a transcoding of the video signal on a number of bits greater than the number of sub-scans is performed.
The restoration of low luminances is improved without it being necessary to increase the number of sub-scans, this latter solution being in any event limited by hardware constraints.